Such a calender is, for example, known from DE-PS 17 29 821. However, the calender here described has not yet been built.
As equalizing measures against the undesired bending of a cylinder, three possibilities are known: the curving of the cylinder, i.e. the convex ball form, the crossing, i.e. crossing the cylinder axes, counter-bending.
In all heretofore delivered packing sheet calenders the pressing cylinders have been ground curved. With other calenders, on the contrary, counter-bending devices and crossing installations have also come into operation.
The curvature is a constant value which is calculated to compensate for the bending with a predetermined loading or determined by practice. Gap force and curvature have a constant relation to one another. It thus results that with another loading, whether higher or lower, corresponding damage to the accuracy of the product results. However, because of variable sheet thickness and very different mixture compositions for different packing sheet use, the practice of packing sheet preparation requires different cylinder gap forces.
As the outer surface of the cylinder is subject to remarkable wear, which is greater in the middle of the cylinder than at the edges, the camber becomes constantly smaller with increased use of the cylinder. With increasing tolerance requirements for the product, the need of an adjustable correction device for the deflection becomes ever clearer.
The crossing of the cylinders is, on technical process grounds, unsuitable for a packing sheet calender.
Thus, of the two variable methods of compensating the deflection, there remains only the possibility of counterbending. Packing sheet calenders generally have two cylinders of different size, namely a large heated working cylinder, on which the packing sheet is constructed and thereby vulcanized and a smaller, usually cooled, pressing cylinder which applies pressure to the packing sheet. A counterbending is conceivable only on this smaller cylinder.
For a long time, the cylinder position of this pressing cylinder has been effected hydraulically, as definite cylinder forces, which are necessary for the sheet production, could not be obtained by mechanical positioning devices, for example with threaded spindles.
The counter-bending devices for the cylinders, which are per se known in calender construction, must, for their function, apply large forces, whereby the support mostly on the calender stands is effected. With the known construction of counter-bending devices, these large forces so greatly impair the cylinder pressure that their practical use in the construction of packing sheet calenders is practically prohibited. For when, with packing sheet calenders of known construction, a hydraulically actuated device for compensating cylinder bending acts on the thin cold cylinder in addition to the hydraulic pressing device, the bearing forces which are applied to the ends of the adjustable cylinder are added to the pressing forces. Through this fault, considerable rejects arise. The indicated value of the pressure measuring device no longer corresponds to the applied cylinder force. The calculation of the actual working pressure on the packing sheet being formed on the large heavy working cylinder is extraordinarily complicated and loaded with considerable uncertain factors.
DE-PS 17 29 821 proposes eliminating these disadvantages in that, contrary to the known and until now usual practice, the thinner and lighter cold cylinder, on which the counter-bending device works, is rotatably supported fast in the calender stands while the heavier and heated working cylinder is adjustable in the calender stand. However, with this construction, the adjustability of the large and very heavy working cylinder relative to the driving force transmission raises serious problems, in particular, syncronizing problems. Moreover, the adjustment of the large heavy working cylinder is disadvantageous in that because of the very high mass of this cylinder, the measuring of the adjustment forces is loaded in an undesirable manner with errors. The incident construction difficulties were so great that this construction of a packing sheet calender has not come into production.
Also known, from the construction of rubber or plastic material calenders is the combined use of cylinder curvature and counter-bending. The counter-bending device is then so constructed that the counter-bending can work in both directions, so as to provide a corresponding compensating adjustment for both larger and smaller gap forces. However, the different force directions of the counter-bending raises considerable problems in the construction of packing sheet presses so that also this combined art has not come into use.